A1000 heatsinks

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Daniel Quinn
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#1 A1000 heatsinks

Post by Daniel Quinn »

I’ve had some class a boards based on musical fidelity a1000 for some time . They are a lot larger than they looked in the photos ( 14 inch by 6 inch ) and require it says a heatsink that is 6kg per board !.

I can’t find 6kg heatsinks and weight is not the best indicator of heat dissipation.

As anybody any idea of Suitable heatsink?
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ed
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#2 Re: A1000 heatsinks

Post by ed »

I'll start by saying that I have no experience of the a1000, although I do have the circuit, without any voltage figures unfortunately.

There were 2 distinct a1000 versions mentioned on the web, and various clones. The original was 2 narrow boards, each with 4 x mj15004 and 4 x mj15003. Some sources say 30w per channel and some say 50w. Nailing down the heat dissipation for your boards would be nigh on impossible without the voltage figures.

an example: www.amazon.co.uk/Musical-Fidelity-MJ150 ... B077794X9S


Heatsinks are usually measure in coefficient and specified as degrees c/watt. I can give you a rough idea based on the sinks I use for my class a amps. I consistently use conrad sinks that are 300mm front to back by 150mm high by 40mm fins. These are specced at 0.25 degrees per watt and weigh 2.5kg. They consistently come in at 45 degrees, which is roughly 25-30 degrees above ambient.

In reality my sinks look slightly bigger than the original sinks fitted to the mf a1000. all my class a amps produce no more than 100w dissipation per channel. I would take your 6kg figure under advisement.
The original A1000 had 400va tx and 28-30v rails so I would guess it produces no more heat than one of mine so a sink with 0.2 degrees or below would be adequate.....

it's a guess but if you had some voltage figures for your board we could give you an exact figure.
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Daniel Quinn
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#3 Re: A1000 heatsinks

Post by Daniel Quinn »

Thanks , most illuminating

It recommends 500va with 24 v secondary per board.

The quiescent current in 500ma

If I plan on the heatsink being less than 60 degrees what kind of heatsink do I need
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#4 Re: A1000 heatsinks

Post by ed »

need more info

is there one board for both channels or two?
how many output devices per channel. I ask because the original had 8 and the amazon example shows 4. How many for each channel on yours?

if the quiescent figure you have given is the bias figure then assuming you have a minimum source resistor to measure and set the bias...
you can multiply the rail voltage minus say 0.25v by the current through the device by the number of devices...
i.e 23.75 x 0.5 x number of devices = watts dissipation per channel

multiply this figure by the coefficient of the heat sink you have which gives the temp of the heatsink above ambient.
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Nick
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#5 Re: A1000 heatsinks

Post by Nick »

So assuming on board per heat sink and one board per channel:

24v transformer will give 1.41 * 24v = 34v

34 * 0.5 = 17w

It will be this for both the +ve and -ve rail transistors so total dissapation will be 34W. Doesn't seem much for a 50W class A amp.

Sp if you have a 0.2 W/C heatsink you would expect 34 * 0.2 or 7C increase over room temperature.
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#6 Re: A1000 heatsinks

Post by ed »

all the a1000 circuits I've seen have 24-28v rails....if it really is 24v secondary then it doesn't relate to any of the boards I've seen....except one I found on ebay...

my dissipation figures are for each device.
My figures were all examples based on the rail voltages I'd seen. I rather thought DQ would adjust for his actual rail voltage.

are my figures incorrect?
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#7 Re: A1000 heatsinks

Post by Daniel Quinn »

it as 8 0utput devices per mono board and its 24v to 28 v ,

is dissipation is 34w in total or per output device?


i have also got the 4vdevice board as well in a cuboard
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#8 Re: A1000 heatsinks

Post by ed »

then I stand by rail voltage(give or take subtraction of any sense resistor) x 0.5 x number of devices.
this for each channel.

multiply this number by the heat sink coefficient to determine the temp of the heatsink above ambient.
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#9 Re: A1000 heatsinks

Post by Nick »

ed wrote: Tue Feb 08, 2022 3:03 pm all the a1000 circuits I've seen have 24-28v rails....if it really is 24v secondary then it doesn't relate to any of the boards I've seen....except one I found on ebay...

my dissipation figures are for each device.
My figures were all examples based on the rail voltages I'd seen. I rather thought DQ would adjust for his actual rail voltage.

are my figures incorrect?
No, I am sure they are correct. As I said (I think) it depends if the quiescent current figure is for the amp or for the device. If you were talking about the power supply then I would (and did) assume total. But I guess its not.

But if it takes 24v to 28v then a 24v transformer will be wrong.
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#10 Re: A1000 heatsinks

Post by ed »

I was applying my usual 2p worth of logic.....if DQ says it wants a 400va Tx then I'd stake your reputation on the fact the figure of 500ma quiescent is for 1 device.

elsewise you might get away with a 100va Tx
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#11 Re: A1000 heatsinks

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ed wrote: Tue Feb 08, 2022 5:42 pm I was applying my usual 2p worth of logic.....if DQ says it wants a 400va Tx then I'd stake your reputation on the fact the figure of 500ma quiescent is for 1 device.

elsewise you might get away with a 100va Tx
Dennis likes to use more va in his amps, so 400va would (probably) not be a necessity.

If I remember correctly, the heatsink on the MF A1 (and A1000 ?) could be used to make toast. :lol:
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#12 Re: A1000 heatsinks

Post by Ray P »

I would de-rate the heatsinks a little - you may not get the the specified heat disipation in a real-world context.
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